1. Field of the Invention
The invention relates to a method for simulation of high-quality daylight spectra at color temperatures in the range of 2,700 K to 10,000 K, particularly for the purpose of lighting matching surfaces in subjectively visual color coordination of a finished product with artwork, in the graphics industry, as well as in other industrial sectors. The invention furthermore relates to a multispectral color coordination system for performing the method according to the invention.
2. Description of the Related Art
In the graphics industry, color-precise reproduction between artwork (proof or original artwork, such as, for example, textiles, plastic parts, painted parts, motor vehicle interior, etc.) and print is an essential prerequisite for meeting the ever greater demands on quality assurance and the requirement to reduce costs.
Aside from quality control using measurement technology, a visual assessment of the color agreement achieved is an important prerequisite.
Lighting plays the greatest influence in visual matching. Incorrect lighting leads to incorrect assessments and thus necessarily to complaints and to increased costs in the production process.
Paints and pigments can visually awaken the same color impression, although they are different in the chemical and/or spectral composition, such as printing inks for offset printing, intaglio printing, digital printing systems on the basis of inkjet printer inks or toner-based systems, paints for the treatment of surfaces, or pigments for dyeing plastics or textiles. Frequently, it is required that these different materials are supposed to be identical in terms of color, for example a product picture in a catalog or the textile interior and the dashboard, made of plastic, in a car.
If these materials are assessed under different light conditions, these can look identical under a specific type of light, and differ completely under a different type of light. This is called metamerism.
In the printing process, for example, artwork for the printed products is increasingly being shown only on the monitor (called a soft proof in the technical language), instead of using a proof as the print artwork, and these soft proofs have already achieved a high quality level both for matching in the printing pre-stage and on the control console of the printing machine.
The LCD monitors used for the soft proof have reached a high color reproduction quality in the meantime, and for this reason, any remaining visible color differences are now ascribed to the color reproduction properties of the lighting used for the color assessment.
Specifically in the case of monitor workstations, in which the print artwork is coordinated and released online at different locations (for example: printing client in Hamburg; print pre-stage in Munich, and printer in Stuttgart) (referred to as remote soft proof in the technical language), this is criticized by manufacturers and users as a disadvantage of these proof systems.
While the known color coordination systems predominantly used for color matching, on the basis of fluorescent lamps, meet the requirements of the currently valid ISO standard 3664 for color matching, they no longer meet the high quality demands of the new proof technologies.
As a typical example, the color matching system according to DE 10 2005 019 135 A1 will be mentioned here; however, the same holds true also for color matching systems of the manufacturers Graphic Technology Inc. (GTI), USA; Verivide, UK, or Xrite, to mention only a few examples.
While fluorescent lamps are an advantageous light source for a color matching cabin of a color coordination system, the spectral distribution of these lamps has multiple peaks, so-called peaks, which are attributable to the different gas discharges in the fluorescent lamp (for example mercury at 546 nm). Thus, the color reproduction cannot be directly assessed by way of a spectral comparison of test light type and reference light type, but rather only using the color impression that results, in each instance. For this purpose, the comparison, tolerances for the color location of the light source, the color reproduction index, and the metamerism index, are therefore generally used.
However, these tolerances are too broad for future applications, and therefore make the current standard light devices usable only with restrictions.
Another problem contains the aging behavior of the fluorescent lamps. The phosphors used in the fluorescent lamps as a fluorescent change their light color with increased aging. Empirical values show that the known fluorescents are suitable for adhering to the current tolerances, which are still very broad, only for 2500 hours of operation.
Since the color location shift is a permanent process, the known fluorescents can be used only for a clearly shorter time for future applications, since the color location shift is very clearly perceived by the human eye.
Furthermore, the question concerning the “right” color temperature is being asked more and more often. Thus, at the present time, the alternative use of the standard light type D65 is already being demanded in many cases; this is already being used as a matching light in many related industries such as the textile or automotive industry.
As has already been mentioned above, the spectrum of a fluorescent lamp is not adjustable. The user is forced to additionally acquire new matching devices for matching under a different type of light.
Furthermore, light sources based on LEDs have been known for several decades. Their long useful lifetime and their robustness are only two of many reasons that predestine them for a multitudinous number of applications in daily life. However, because of their spectral properties, LEDs are not suitable, even today, for applications for the purpose of visual color assessment.
The spectrum of white LEDs is very incomplete, and therefore the color reproduction properties are unsuitable for the applications in visual color matching that have been described. For example, the color reproduction index clearly lies below the required value of 90.
Various approaches for simulating daylight on the basis of the R-G-B technology, by means of additive color mixing with red (R), green (G), and blue (B) LEDs, something that would theoretically be possible, are known.
However, natural daylight has a uniform spectral progression from 380 nm to 780 nm, and furthermore, UV components are contained in daylight.
However, since colored light-emitting diodes do not cover a spectrum, but rather produce light only at a specific wavelength, while it is true that all possible light colors can be produced with RGB LEDs, only very incomplete simulation of daylight is possible by means of the combination of three-color LEDs.
The color reproduction properties are therefore limited, and for this reason, all attempts to produce high-quality daylight simulation for visual color assessment according to the standards ISO 3664 or DIN 6173, for example, on the basis of LED technology, have failed.
There are currently many other applications with LEDs, but there, the spectral properties of the light are not important.
A system according to DE 10 2006 003 257 A1, a device for detecting optical reference marks, could be mentioned here as an example. This system comprises a lighting device having LED chips, which are affixed to a circuit board, together with a camera, and the LED chips are used to illuminate the positioning field of the camera.
In this application, all that is important is the uniformity of the illumination of the positioning field, whereby no spectral properties of the light are specified.
Another application of LED lamps is a dental treatment lamp for producing a light field for treatment with a dental material that is cured by light, for example according to DE 10 2006 038 504 A1. White and colored LEDs or exclusively colored LEDs are used as a light source, in order to illuminate the oral cavity with a white light field having a color temperature of 3600 to 6500 K and an illumination intensity of 8000 lx. The color reproduction index Ra according to CIE (Commission internationale de l'eclairage) 13.3 lies at about 85.
DE 10 2004 049 604 A1 discloses a lighting system for a printing machine with LEDs having the colors red, green, and blue. This lighting system is used to better detect the changes in color, behavior, and contrast on the imprinted material in the delivery of the printing machine than is possible under white-light conditions. The disadvantage of this system is, as has already been stated above, that no high-quality daylight simulation is possible when using only LEDs in the primary colors red, green, and blue.
With EP 1 314 972 A1, a spectral photometer or color densitometer and its use are disclosed; in particular, only a small measurement spot is illuminated with this device. This device is not suitable for spectral measurement of colors.
Finally, another device for lighting is disclosed with WO 2007/083250 A1, in which the behavior of the light source is monitored and corrected, but for the purpose that light sources of low quality and/or useful lifetime can be used, which can be unimportant in one application or another, where quality requirements are low, but these cannot be used for the applications mentioned initially. Furthermore, this device has a complicated technology.